This is (mostly) covered in post 1 of this thread. Also in a couple other Hornresp tutorials and in the Hornresp Help file. And for future reference you can just make changes and check the Schematic to see what happened and determine what each input box does. I strongly recommend reading all the tutorials you can find and also the Hornresp Help file as a bare minimum. Anyway...

Vrc - volume of the rear chamber in liters
Lrc - length of rear chamber in cm
Fr and Tal - are concerned with stuffing in the rear chamber which isn't usually used, these are usually left blank for front loaded horns. Also, double clicking will change Fr and Tal to other inputs that also are not usually used.
Vtc - volume of throat chamber in cubic cm
Atc - area of throat chamber opening in sq cm

As mentioned Le is VC inductance and it isn't incredibly important because there's no accurate way to model inductance unless you use a much more complicated calculation than Hornresp uses. But I use it anyway and if the manufacturer doesn't list it, it can be estimated based on Le values for other similar drivers (similar in xmax values and inductance control features or lack thereof) and the value of Re.